This is according to a FBI report that will be released in September 2016. It is known the staffer whose account gets breached is female, but her name will be redacted. The unnamed hacker uses the anonymity software Tor to browse through this staffer’s messages and attachments on the server.

The FBI will call this the only confirmed “successful compromise of an email account on the server.” But the FBI will not be able to determine who the hacker is or how the hacker obtained the staffer’s username and password to access her account. (Federal Bureau of Investigation, 9/2/2016)

Wired will later comment, “The compromise of a Bill Clinton staffer—who almost certainly had no access to any of then-Secretary Clinton’s classified material—doesn’t make the security of those classified documents any clearer. But it will no doubt be seized on by the Clintons’ political opponents to raise more questions about their server’s security.”

Dave Aitel (Credit: Immunity)

Clinton’s computer technician Bryan Pagliano is in charge of monitoring the server’s access logs at the time.

But Dave Aitel, a former NSA security analyst and founder of the cypersecurity company Immunity, will later comment that the breach shows a lack of attention to the logs. “They weren’t auditing and restricting IP addresses accessing the server. That’s annoying and difficult when your user is the secretary of state and traveling all around the world… But if she’s in Russia and I see a login from Afghanistan, I’d say that’s not right, and I’d take some intrusion detection action. That’s not the level this team was at.” (Wired, 9/2/2016)

When Pagliano is interviewed by the FBI in December 2015, he will claim that he knew of no instance when the server was successfully breached, suggesting he didn’t know about this incident. (Federal Bureau of Investigation, 9/2/2016)

And when Justin Cooper, a Bill Clinton aide who helped Pagliano manage the server, will be asked about the incident in September 2016, he will say he knew nothing about it until he read about it in the FBI report released earlier that month. (US Congress, 9/13/2016)